Hydrobiologia 430: 121–147, 2000.T.J. Pandian (ed.), Recent Advances in Indian Aquatic Research.© 2000Kluwer Academic Publishers. Printed in the Netherlands.

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Ecology and fishery management of reservoirs in India

V. V. SugunanCentral Inland Capture Fisheries Research Institute, Barrackpore 743101, West Bengal, India

Key words: limnology, primary productivity, energy transformation, culture-based fisheries, enhancement,stocking, exotic species

Abstract

India has 19 370 small reservoirs with a total water surface area of 3 153 366 ha. At least 100 of them havebeen subjected to scientific studies. Habitat variables responsible for a reservoir’s productivity can be summed upinto climatic, morphometric and hydro-edaphic factors. The peninsular reservoirs are characterized by a narrowrange of fluctuations in water and air temperature across seasons, a phenomenon which prevents the formation ofthermal stratification. Many reservoirs in the upper peninsula show thermal stratification during summer. Wind-induced turbulence facilitates the return of nutrients to the trophogenic zone. Most reservoirs on the mountainslopes of Western Ghats, Himalayas and the other highlands are deeper, with steeper basin walls, compared toirrigation impoundments. Mean depth does not show any direct correlation with productivity, either at primaryor fish level. A high shoreline development index gives a better indication of productivity. Plankton, benthosand periphyton pulses of Indian reservoirs coincide with the months of least level fluctuations. Oligotrophictendencies shown by some reservoirs are mainly due to poor nutrient status and other chemical deficiencies. Inmost cases, poor water quality is accountable to poor catchment soil. Low levels of phosphate and nitrate arenot indicative of low productivity due to quick recycling of these nutrients. Specific conductivity reflects theproduction propensities of reservoirs satisfactorily. Almost all productive reservoirs have a klinograde oxygencurve and a vertical stratification of chemical variables such as pH, carbon dioxide, total alkalinity and specificconductivity. High seasonal rainfall and discharge of water during monsoon result in high flushing rates, which donot favour colonization by macrophytic communities. Similarly, inadequate availability of suitable substrata retardsthe growth of periphyton. Plankton constitutes the major link in the trophic structure. A rich plankton communitywith well-marked succession is the hallmark of Indian reservoirs with blue-green algae as the major component.The main factors that retard the growth of benthos are a rocky bottom, frequent water level fluctuation and rapiddeposition of silt and other suspended particles. Large reservoirs, on average, harbour 60 species of fishes, of whichat least 40 contribute to the commercial fisheries. Fast-growing Indian major carps are the prominent commercialfishes. Dam construction has adversely affected populations of many other species such asTenualosa ilisha, Torspp. andCirrhinus spp. Formulae for estimating fish yield potential and stocking density are described. Whileculture-based fisheries have been successfully practiced in many small reservoirs, the management norm followedin medium and large reservoirs is primarily on capture fishery. In large and medium reservoirs, stocking wassuccessful only when stocked fishes bred. Indian experience on species enhancement and introductions is described.Environmental enhancement of small reservoirs has been attempted in some reservoirs of Tamil Nadu. Modeling,using standard population parameters, such as thedensity-dependent growth, size dependent mortalityandweight–length relationshipis discussed. Two exotic fishes viz.,Oreochromis mossambicusand Cyprinus carpiohavebeen introduced into Indian reservoir with discouraging results.Hypophthalmichthys molitrix, after an accidentalintroduction, has performed well in Gobindsagar, a reservoir with a distinct cold water regime. Reservoir fisheriesin India are well poised for development, provided scientific management norms are adopted.

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Introduction

A large number of reservoirs have been constructed inIndia during the last five decades, with the primary ob-jective of storing river water for irrigation and powergeneration. Although these water bodies hold tre-mendous fisheries development potential, they are notcontributing to the inland fish production of the coun-try to the extent they should. Unlike rivers, which areunder increasing threat of environmental degradation,reservoirs offer ample scope for fish yield optimizationthrough suitable management. The sheer magnitudeof the resource makes it possible to enable substantialincrease in production by even a modest improvementin yield. Thus, any attempt to increase productivity ininland fisheries has to rely heavily on reservoirs.

Geo-climatic situations in India make it imper-ative to store river water behind dams. The majorphysiographic divisions of the country are Himalayas,the Indo-Gangetic plains, the Vindhyas, the Satpuras,the Western Ghats, the Eastern Ghats, coastal plains,the deltas and the riverine wetlands (Figure 1). Thealignments of hills and their elevation have a pro-found influence on the prevailing winds and therebythe distribution of rainfall in the country. India re-ceives, on average, 105 cm of rainfall every year,which is one of the highest in the world for a coun-try of comparable size (Rama, 1978). Total amountof annual rainfall is estimated at 400 million hectaremeters (mhm) out of which 230 mhm goes back to theatmosphere as evapo-transpiration, leaving 170 mhmto the river (Rama, 1978). The temporal and spatialdistribution of rainfall exhibits wide variations. Morethan 1 million km2 of the country receives inadequaterainfall (Rao, 1979). This includes deserts, the semi-arid regions of north India and the rain shadow of theWestern Ghats (Figure 2).

Large peninsular rivers like the Godavari, theKrishna, the Pennar and the Cauvery pass throughextensive tracts of low rainfall area and hence carrymuch less water than rivers passing through high rain-fall areas like the northeast and the West Coast. In thenortheast, the Khasi and Jaintia hills receive a bounti-ful 1000 cm of rainfall annually and the Brahmaputravalley gets 200 cm of precipitation. Rainfall up to1142 cm recorded in Cherrapunji and Mawsyngram isone of the highest in the world (Rao, 1979); the westcoast of India, the Indo-Gangetic plains and the Him-alayas receive rainfall of high order during southwestmonsoon.

Southwest monsoon (June–September) is the prin-

cipal rainy season, when 70% of annual rainfall isreceived. In more than one third of the country, 90%of the rainfall and thereby surface flow is limited toa period of 2–3 months. This extreme seasonalitymakes irrigation reservoirs asine qua nonfor agri-culture, especially in the rain shadow of peninsularIndia. People inhabiting this area store water by erect-ing barricades across minor stream and rivulets fromtime immemorial. In recent years, with the advent ofmodern hydraulic structures, larger and more complexdams came into being. The steep gradient and heavydischarge of water from the mountain slopes of theWestern Ghats, the northeast and the Himalayas offeropportunities for hydroelectric power generation. Alarge number of such projects have come up in theseregions in recent years. Thus, reservoirs have becomea common feature in the Indian landscape dotting allriver basins, minor drainages and seasonal streams.

Definition and classification of reservoirs in India

A reservoir is an impoundment obstructing the surfaceflow of a river, stream or any water course (Sug-unan, 1997a). However, water bodies less than 10 hain area have been excluded form this definition. TheMinistry of Agriculture, Government of India classi-fies reservoirs as small (<1000 ha), medium (1000 to5000 ha) and large (>5000 ha) for purpose of fish-ery management, which constitute the single largestinland fisheries resource in terms of resource size andproduction potential.

Medium and large reservoirs are fewer in num-ber and details on them are readily available with theirrigation, power and public works authorities. How-ever, enumeration of small reservoirs is a tedious task,as they are ubiquitous and numerous. There also ex-ist ambiguities in the nomenclature followed by somestates. The wordtankis often loosely defined and usedin common parlance to describe small irrigation reser-voirs. In the eastern states of Orissa and West Bengal,pond and tank are interchangeable expressions, whilein Andhra Pradesh, Karnataka and Tamil Nadu, tanksrefer to a section of irrigation reservoirs includingsmall and medium sized water bodies.

Tanks, as defined by David et al. (1974), are “waterbodies created by dams built of rubble, earth, stonemasonry work across seasonal streams as againstreservoirs formed by dams built with precise engin-eering skill across perennial or long seasonal riversor streams using concrete masonry or stone for power

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Figure 1. Physiographic divisions of India.

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Figure 2. Distribution of rainfall in India.

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supply large scale irrigation or flood control pur-poses.” This definition is obviously tedious and in-adequate. From limnological and fisheries point ofview, the distinction between small reservoirs andtanks seems to be irrelevant. Moreover, numeroussmall reservoirs fitting exactly the description of southIndian reservoirs are already enlisted as reservoirs inthe rest of the country. Therefore, large tanks need tobe treated at par with reservoirs.

In Andhra Pradesh, tanks are classified asper-ennial and long seasonal. In Tamil Nadu, tanks areclassified asshort seasonaland long seasonal. Thelatter, also known as major irrigation tanks, havean average size of 34 ha and retain water for 9–12months. Similarly, 4605 perennial large water bodiesin Karnataka listed as major irrigation tanks could beeasily brought under the ambit of reservoirs. After re-moving these anomalies in nomenclature, it has beenestimated that India has 19 134 small reservoirs with atotal water surface area of 1 485 557 ha. Similarly, 180medium and 56 large reservoirs of the country have anarea of 527 541 and 1 140 268 ha, respectively (Figure3). Thus, the country has 19 370 reservoirs covering3 153 366 ha (Sugunan, 1995).

Review of work done in Indian reservoirs

Considering the number and surface area of reservoirsin India and their importance in fishery development,research on this resource can be qualified as modest.About 100 reservoirs have been subjected to someform of studies. Research work done in various statesis briefly outlined in Table 1.

Factors determining productivity

The water and soil quality in a reservoir is a func-tion of geo-climatic conditions. Productivity dependson the synergistic effects of geo-chemical, meteorolo-gical, morphometric and hydrographic variables. In-dian reservoirs are spread over various types of terrainand soil types, exposed to diverse climatic conditionsand they receive drainage from a variety of catchmentareas. This imparts a high degree of diversity to theirbiotic communities in terms of biomass and speciesnumber.

In reservoirs, fluviatile and lacustrine charactersco-exist. The lotic sector of the reservoir sustains afluviatile biocoenos, whereas the lenitc and bay zones

harbour lacustrine communities (Sugunan, 1991).During the months of heavy inflow and outflow, thewhole reservoir mimics a lotic environment, whereasin summer, when inflow and outflow dwindle, a lenticcondition prevails. Another unique feature of reser-voirs is the water renewal pattern, marked by swiftchanges in level. In India, most precipitation is duringmonsoon. During this period, due to heavy inflow, alloutlets of the dams are usually opened, resulting intotal flushing. This process dislodges a considerablepart of the biotic communities and disturbs naturalcommunity succession (Sugunan, 1980). Sudden levelfluctuations also affect the benthos by exposing orsubmerging substrata (Sugunan & Das, 1983) .

In reservoirs, nutrient input from allochthonoussources often determines water quality, nutrient re-gime and production. Catchment of parent rivers isoften situated far away from the reservoir, undertotally different geo-climatic conditions. Deep draw-down, wind-mediated turbulence and locking up ofnutrients in deep basins are but some factors thatimpart uniqueness to the reservoir ecosystem. Be-sides, the varying purpose and design of the damsmake reservoirs different in their hydrographic andmorpho-edaphic characteristics, with implications onproduction.

Construction of a dam and consequent impound-ment bring a sudden transformation of a lotic envir-onment to a lentic one. This process triggers a seriesof changes in the riverine community, which are akinto secondary community succession. A number oforganisms perish, some migrate to more hospitableenvirons, and the more hardy ones adapt themselvesto the changed habitat. There is usually an initialspurt of plankton and benthic communities due to theincreased availability of nutrients released from thedecay of submerged vegetation. This trophic burst isalso on account of the saproxenic lacustrine speciesfilling the vacant niches created by the disappear-ance of saprophobic riverine taxa. As the effects oftrophic burst wean away, the reservoir passes into aphase of trophic depression and the final fertility is re-gained after a few years. Habitat variables responsiblefor a reservoir’s productivity can be summed up intoclimatic, morphometric and hydro-edaphic factors.

Climatic factors

The Indian reservoirs are distributed in a wide rangeof climates extending from the temperate Himalayasin the north to the tropical in the southern peninsula.

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Figure 3. Distribution of reservoirs in India.

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Table 1. Summary of scientific literature on reservoir fisheries of India

State/Reservoir Nature of work Reference

Tamil Nadu

Red Hills lake Plankton, water quality Ganapati (1940)

Stanley reservoir Limnology and fisheries Ganapati &Alikunhi (1949), Ganapati

(1955), Sreenivasan, (1966, 1969)

Hope Lake Limnology and fisheries Ganapati (1956a)

Errakuppam Limnology Ganapati (1956b)

Mukerti reservoir Limnology Ganapati (1957)

Bhavanisagar Limnology and fisheries, post- Sreenivasan (1964),Sreenivasan et al.

impondment changes, phytoplankton, (1964), Dorairaj & Pankajam (1956),

Biology of Puntius dubius, Aorichthys Menon & Chari (1959), Franklin

aor, Labeo calbasuandChanna (1969), Ranganathan et al. (1962),

marulius,Fisheries management Ranganathan & Radha (1966),

Natarajan (1971), Devaraj (1973),

Natarajan et al. (1981)

Amaravathy Limnology and productivity Sreenivasan (1965)

Tamil Nadu Comparative limnology Sreenivasan (1970a,b, 1976, 1979)

reservoirs Pandian (1987)

Poondi reservoir Sounder Raj et al. (1971)

Aliyar, Fisheries management Selvaraj et al. (1997)

Tirumoorthy

Kerala

Idukki Fish fauna, water quailty, plankton, Gopinath & Jayakrishnan (1984),

Khatri (1985, 1987, 1988), Nair (1988)

Parappar Nair (1986), Sahib &Aziz (1989)

Neyyar Harikrishnan &Aziz (1989)

Kerala reservoirs Fisheries management Mathew & Mohan (1990)

Malampuzha, Stock assessment, fisheries management Taege et al. (1993)

Chulliar, Pothundy,

Vazhani and Peechi

Karnataka

Tungabhadra Limnology, plankton, benthos, Fisheries, David et al. (1969, 1975), Govind

fishing gear (1963, 1969), Krishnamoorthy (1966),

Subba Rao & Govind (1964), Banrerjee

& Ray (1979) Singit (1987), Singit et

al. (1987)

Hemavathy Limnology and fisheries Devaraj et al. (1987)

Krishnarajasagar Limnology and fisheries Sugunan (1995)

Nalliguda

Continued on p. 128

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Table 1. contd.

Markonahalli Limnology and fisheries Anon. (1998)

Supa Preliminary observations Birasal et al. (1991)

Vanivilas Sagar Preliminary observations Ray (1969)

Kabini Preliminary observations Murthy et al. (1986), Srivastava et al.

(1985)

Byramangala Pollution, fish mortality Raghavan et al. (1977), Joshi (1990)

Andhra Pradesh

Hussainsagar Water quality, phytoplankton, heavy Srinivasan et al. (1965), Zafar (1966,

metal pollution, comparison with other 1986), Seenayya & Prahlad (1987),

reservoirs Prahlad & Seenyya (1988), Prasad,

(1993), Ghosh & George (1989)

Nagarjunasagar Water quality and primary productivity, Anon (1982), Pathak (1979), Sugunan

plankton, benthos and periphyton, (1980, 1991), Sugunan & Das (1983),

biological traits of various species of Sugunan & Pathak (1986), Sugunan &

fish Vinci (1981), Vinci (1984, 1986), Vinci

& Sugunan (1981)

Madhya Pradesh

Gandhisagar Pre-impoundment survey, post-impoundment Dubey & Mehra (1959), Dubey &

studies, trawling, Chatterjee (1976) Rao et al. (1990),

Kartha & Rao (1990)

Ravishankarsagar, Limnology and fisheries Sugunan, 1995

Govindgarh Limnology and fish productivity Mathew (1975)

Kulgarhi Limnology and fisheries Dwivedi et al. (1986), Karamchandani

& Mishra (1980)

Mansarovar Water quality, plankton and fish fauna Kulshreshtha et al. (1992)

Loni Fishery biology Gupta (1976)

Bergi,Tawa,Barna, Preliminary observations Unni (1993)

Sarni,Sukta,

Sampan,

Kolar,Halali, Dahod

Undasa Preliminary observations Singh (1986)

Yaswantnagar Preliminary observations Sharma & Diwan (1989)

Orissa & Maharashtra

Hirakud Water quality, productive potential and Sugunan & Yadava (1992)

the management, craft, gear and fishing George(1979), Sulochanan et al. (1968),

methods, ecology Nair et al. (1981), Khan et al. (1992),

Varghese et al. (1993), Dash et al.

(1993)

Dhom Preliminary studies Trivedi (1993)

Continued on p. 129

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Table 1. contd.

Nathsagar Preliminary studies Desai (1980)

Bhandardara, Preliminary studies Valsangkar (1980, 1987, 1993)

Yaldari and Girna,

Bhatgar Fisheries management Anon. (1997)

Gujarat & Rajasthan

Ukai Ecology and fisheries Anon. (1980, 1983, 1984a)

Sayaji Sarovar Preliminary studies Ganapti & Pathak (1969)

Jaisamund Limnology and fisheries Durve & Kakkar (1982), Sharma

(1980)

Ramgarh and Water quality, fish production potential Jhingran (1989)

Chhapparwara

Himachal Pradesh

Gobindsagar Water and soil quality fish and fisheries, Sarkar et al. (1977) Kaushal & Rao

fish biology, gill netting (1982, 1990), Kaushal et al. (1980 a,b),

George et al. (1977)

Pong Limnology and fisheries CIFRI reports (Sugunan, 1995)

Pandoh Preliminary survey CIFRI reports (Sugunan, 1995)

Uttar Pradesh

Rihand Ecology and fisheries Natarajan et al. (1982)

Gularia Ecology and fisheries Jhingran et al. (1981)

Bachhra Ecology and fisheries Khan et al. (1990)

Baghla Ecology and fisheries Sugunan (1995)

Baigul Preliminary survey Khan (1986), Salim & Ahmed (1985)

Keetham Preliminary survey Dwivedi & Chonder (1980)

Bihar

Konar, Tilaiya, Limnology, fish productivity, fisheries Jhingran & Natarajan (1969), Natarajan

Maithon and management (1976), Ramakrishniah & Sarkar

Panchet (1982)

Getalsud Fisheries management, plankton, Anon (1984b), Singh (1984)

benthos

Bihar reservoirs Resource survey Ahmed & Singh (1992)

Badua & Nalkari Preliminary observations Verma & Munshi (1983), Sarkar,

(1982)

The northeast

Gumti Water quality, fish production trends Chaudhuri (1992), Anon. (1994)

Kyrdemkulai & Limnology, productivity guidelines for Sugunan & Yadava (1991 a , b).

Nongmahir fisheries management

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Table 2. Latitude-induced variations in availability of energy inIndian reservoirs (from Jhingran, 1990)

Reservoir Area (ha) Latitude Available Availableat full (N) light energy chemical energy

reservoir level (cal m−2 yr−1 (cal m−2 yr−1

Bhavanisagar 7285 11◦ 25′ 213× 04 8781 (0.41%)Nagarjunasagar 28 474 16◦ 4′ 205×104 5959 (0.29%)Rihand 46 538 24◦ 188×104 3970 (0.20%)Ramgarh 1265 27◦ 12′ 183×104 8236 (0.49%)Gobindsagar 16 867 31◦ 25′ 172×104 11 696 (0.68%)

Apart from influencing the prevailing climate of theregion, the latitudinal location is important in determ-ining the quantum of solar radiation available at thewater surface for primary productivity. Natarajan &Pathak (1983) estimated the amount solar radiationavailable at four reservoirs within 11◦ 25′ N and 31◦25′ N and the rate, at which the solar energy wasconverted into chemical energy. Incident solar energyavailable at four reservoirs varied from 213× 104 calm−2 yr−1 in Bhavanisagar (11◦ 25′ N) to 172× 104

cal m−2 yr−1 in Gobindsagar (31◦ 25′ N). Jhingran,(1990) observed that 0.29–0.68% of the incident solarenergy was fixed as chemical energy by the primaryproducers in five reservoirs, viz., Gobindsagar (Hi-machal Pradesh), Ramgarh (Rajasthan), Rihand (UttarPradesh), and Bhavanisagar (Tamil Nadu). It is oftenthe qualitative and quantitative abundance of the pro-ducer communities that determines the photosyntheticefficiency rather than the actual amount of solar en-ergy available. For instance, Nagarjunasagar, despitereceiving solar energy at the rate of 205× 104 cal m−2

yr−1, fixes chemical energy to the extent of 0.29%,whereas in Gobindsagar, 0.68% of the 172× 104 calm−2 is fixed by the producers in an year (Table 2).Seasonal variations in morphometry also play a vitalrole in determining the rate of energy fixation and itstransformation. This aspect has been brought to focusby Haniffa & Pandian (1978) through studies in a pondecosystem.

Air temperature, wind velocity and rainfall aresome predisposing factors in biological productivity ofreservoirs. In contrast to the northern reservoirs, theirpeninsular counterparts are characterised by narrowrange of fluctuations in water and air temperature dur-ing different seasons, a phenomenon, which preventsthe formation of thermal stratification. Nagarjunas-agar is a classical example, where no thermocline isformed, despite 40% of its capacity being dead stor-

age (Pathak, 1979). Besides, high water temperatureprevailing throughout the year, continuous drawdownfrom deeper layers and wind-mediated turbulence fa-cilitate mixing up of water column. This is equallytrue to most of the reservoirs in the States of AndhraPradesh, Karnataka, Kerala, Tamil Nadu, Orissa andMaharashtra. Thermocline is limnologically importantbecause in thermally stratified lakes, water at surfaceand bottom does not mix up; hence rich nutrients getlocked up at the bottom layer. A warm bottom layeralso facilitates rapid decomposition of organic matterand accelerates the nutrient release.

Seven reservoirs in the upper peninsula compris-ing south Bihar, Gujarat and Madhya Pradesh undergotransient phases of thermal stratification during sum-mer stagnation, depending upon other parameters suchas basin depth, water abstraction pattern and wind(Sugunan,1995). Konar reservoir, situated above theTropic of Cancer, has distinct epi-and hypolimnionduring summer (Natarajan, 1976). Similarly, a well-defined thermocline is reported from Gobindsagar(Sarkar et al., 1977; Anon., 1989). In this reservoir,apart from the solar warming of the top layer, whichremains as a separate thermal regime, the inflowingBeas water that joins the reservoir at the lotic sectordoes not mix, retains its cool character and remains asa separate layer at the bottom.

Amount of rainfall plays a crucial role in renew-ing water and enriching nutrients of reservoirs. Moreoften, rainfall in catchment of the river situated hun-dreds of km away from the reservoir affects the inflowrate. Another important climatic factor with implic-ations on thermal and chemical regimes of reservoiris the wind. It helps distribution of heat and equaliz-ation of temperature in water column. Wind velocityis high during monsoon and pre-monsoon in mostreservoirs of India (Natarajan, 1979a). Wind-inducedturbulence is important in churning of the reservoirsand thereby facilitating availability of nutrients attrophogenic zone.

Morphometric factors

Reservoir morphometry is a function of dam heightand topography of impounded areas. Nature of the ter-rain, on which dam is constructed, plays a crucial rolein determining the reservoir morphometry. Design ofthe dam and water use pattern also decide productivityby affecting morphometric and hydrographic features.Most hydel reservoirs on mountain slopes of the West-ern Ghats, Himalayas and other highlands are deeper,

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with steeper basin walls, compared to the irrigationimpoundments. An important morphometric consider-ation is the mean depth, which is believed to determineproductivity of reservoirs. This is based on the well-known dictum (Rawson, 1952; Hayes, 1957) thatshallower lakes have greater part of their water in eu-photic zone, facilitating greater mixing and circulationof heat and nutrients, and hence higher productivity. Alarge portion of water in deep lakes serves as anutrientsinkat the bottom, where organic matter accumulatesand becomes unavailable at photosynthetic zone.

Among large reservoirs, mean depth ranges from5.2 m in Panchet to 58 m in Gobindsagar. Me-dium reservoirs have mean depth ranging from 2.3m (Poondi) to 24 m (Bhatghar). Small reservoirshave mean depth range of 2.1 m (Vidur) to 14.6 m(Badua). Hope lake in Tamil Nadu has an excep-tionally deep basin of 37.7 m. Hydel reservoirs ofthe mountain slopes invariably are deeper than theirrigation reservoirs of the plains and plateaus. Incid-entally, the two largest impoundments of the countryviz., Hirakud and Gandhisagar have very low meandepths of 11.3 and 11.7 m, respectively. Idukki, ahydel reservoir in the Western Ghats, which is onetenth of Gandhisagar in area, has a mean depth of 32m. This parameter, however, does not show any dir-ect correlation with productivity, either at primary orfish level. Despite being very shallow, Vidur does notsupport a rich plankton community. Likewise Kulgarhiand Gobindgarh reservoirs in Madhya Pradesh exhibitoligotrophic propensities in spite of their shallowness.On the other hand, Gobindsagar, the deepest reservoirhas the highest productivity among large reservoirs.Medium reservoirs like Amaravathy (13.7 m), Aliyar(16.8 m) and Thirumoorthy (11 m) develop regularblooms of plankton.

Shoreline and volume development indicesAn irregular shoreline encompasses more littoralformations and areas of land and water interface. Thus,a high value of shoreline development index is be-lieved to be indicative of productive nature of thewater body. High shoreline indices of Hirakud (13.5),Gobindsagar (12.26), Tilaiya (9.12), Konar (8.78),Nagarjunasagar (7.89) and Rihand (7.04) are accom-panied by a moderate to rich plankton community.Ratio between the maximum depth and mean depth,often described as volume development index, denotesthe depth of basin in relation to the nature of basinwall. An index value less than 1 suggests basin wallconvex towards water. No perceptible relation exists

between this parameter and the productivity of Indianreservoirs.

Hydrodynamics

Rate of inflow, outflow and water level have a dir-ect bearing on productivity, as their sudden fluctu-ations directly affect the biotic communities. Plank-ton, benthos and periphyton pulses coincide with themonths of least level fluctuations and all these com-munities are at their ebb during the months of max-imum level fluctuations and water discharge (Sugunan,1980; Sugunan & Das, 1983; Sugunan & Pathak,1986). Percentage of shallow areas (littoral forma-tion), which varies at different levels, depending oncontour, is also an indicator of productive nature ofthe lakes. The requirements of irrigation, power gen-eration and other primary purposes of the dam dictatestorage and release of water from dams, rather thanany considerations related to fisheries. The spillwaydischarge, apart from dislodging the standing cropof plankton, removes the oxygenated clear water atthe top layer, leaving the oxygen-deficient bottomwater. Conversely, the deep drawdown removes thedecomposing material including nutrients (Jhingran,1975).

Hydro-edaphic factors

Low productive nature of some of the reservoirs canbe attributed to poor nutrient status. In most cases,deficiencies in the catchment soil result in poor waterquality. Reservoirs in Kerala such as Idukki (Khatri,1985, 1987, 1988), Neyyar (Harikrishnan & Aziz,1989) and Parappar (Nair,1986) have low status interms of specific conductivity (< 50µs) and total al-kalinity (<50 mg l−1) with the attendant low primaryproductivity and plankton. The rivers of Kerala drainWestern Ghats with lateritic and humus soils defi-cient in N, P and Ca. In Tamil Nadu, Hope lake,Manimuthar, Pechiparai and Peruchani are deficient inions, while Sathanur, Krishnagiri and Vidur reservoirsreceiving drainage passing predominantly through cul-tivated area have higher levels of alkalinity and hard-ness (Sreenivasan, 1970a,b, 1976, 1979). Limestoneand other calcareous rocks underlying the water coursein the Deccan plateau are responsible for the predom-inantly hard water character of many of the reser-voirs on the Krishna and Cauvery in Andhra Pradeshand Tamil Nadu. In Madhya Pradesh, the water issoft to medium soft with less mineral salts due to

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geo-chemical reasons. The examples are Ravishankar-sagar (Anon, 1984a), Mansarovar (Kulshreshtha et al.,1992), Loni (Gupta, 1976), Bergi, Tawa, Barna, Sarni,Sukta, Sampna, Halali, Kolar Dahod (Unni, 1993),Undasa (Singh, 1986) and Yaswantnagar (Sharma &Diwan, 1989). Catchment of Ravishankarsagar com-prises rocky, denuded forests and upstream riversare intercepted by impoundments, which further de-prive the water of suspended matter. Allochthonousenrichment with minerals and nutrients of the reser-voir is very low resulting in low standing crop ofplankton. Even small lakes with shallow bottoms likeGovindgarh (Mathew, 1975) do not show signs ofproductivity due to poor chemical make up of thecatchment. Soils in Madhya Pradesh are normallydeep black, medium black, shallow black, mixed redand skeletal, low in nitrogen and phosphorus. Acidicnature of water of the northeastern reservoirs, Kyr-demkulai, Nongmahir (Sugunan & Yadava, 1991a,b)and Barapani is attributable to acidic soil of the reser-voir bed and catchment.

Nutrient status

Low levels of phosphate and nitrate characterize mostof the Indian reservoirs. Phosphate very seldom ex-ceeds 0.1 mg l−1 in reservoirs free from pollution.However, the reservoirs of Rajasthan have particularlyhigh levels of phosphate ranging from traces to 0.929mg l−1 (Sharma, 1980); they receive phosphate fromthe rain washings derived from brown hills, red andyellow and desert soils. In a highly eutrophic reservoirof Mansarovar in Madhya Pradesh, phosphate levelsof 4–13 mg l−1 were recorded (Kulshreshtha et al.,1992). Nitrate nitrogen in water is mostly in traces andseldom exceeds 0.5 mg l−1. Low nutrient level in wa-ter, especially nitrate and phosphate does not indicatelow productivity. In many cases, despite their virtualabsence, production processes are not hampered. InAmaravathy, Bhavanisagar, Gandhisagar, Ravishank-arsagar and many other reservoirs, moderate to veryhigh primary productivity is reported, although phos-phate in water is either absent or present in a very lowconcentration. In tropical reservoirs, phosphate levelin water has limited scope as an indicator of product-ive traits. This phenomenon is attributed to turn overof nutrients (Ehrich, 1960; Abbot, 1967) and theirquick recycling, as seen from the high metabolic rates.Phytoplankton is known to take up 95% of the phos-phorus within 20 min, while some algae could convert

inorganic phosphate into organic state in less than oneminute (Hayes & Phillips, 1958).

A measure of total dissolved solids in the formof total alkalinity or the specific conductivity is abetter indicator of production propensities of a reser-voir in India. A possible exception is the Amaravathyreservoir, which despite very low levels of specificconductivity (38–63µS), total alkalinity (7–84 mgl−1) and total hardness (18–50 mg l−1), supports avery rich plankton community and a good stock offish. The ranges of notable abiotic factors indicatingtheir productivity status of Indian reservoirs are givenin Table 3. A close examination of physical and chem-ical data pertaining to more than 100 reservoirs ofIndia suggests that none of the morphometirc, edaphicand water quality parameters can be used as a depend-able yardstick to predict organic productivity to anydegree of accuracy. Production propensities of eachreservoir are determined by a variety of factors. Ver-tical gradient of dissolved oxygen, however, conveysthe status with a higher level of accuracy.

Klinograde distribution of oxygen is a good indic-ator of productivity, as oxygen is consumed duringthe process of decomposition of organic matter at thebottom. An increase in oxygen at the trophogenic up-per zone indicates the high rate of photosynthesis.Irrespective of their geographic location, almost allproductive reservoirs in India have a klinograde oxy-gen curve. In most cases, the oxycline is accompaniedby a vertical stratification of other chemical para-meters such as pH, carbon dioxide, total alkalinityand specific conductivity. The tropholytic zone hasa steady supply of free carbon dioxide, which reactswith carbonate to produce bicarbonates. This results inan increase of bicarbonates towards the bottom. Sim-ilarly, pH drops rapidly due to increase in hydrogenions. Thus, increase in total alkalinity, specific con-ductivity and CO2 and decrease in pH values towardsthe bottom layers act as useful indicators of productiv-ity. Primary productivity in reservoirs is very high dueto warm conditions prevailing in most parts of India.Many workers consider 1% of total carbon produc-tion at the phytoplankton phase to represent relativelyhigher potential fish production from a water body,although almost all reservoirs produce much less fishthan their potential.

Biotic communities

Heavy discharge of water during the monsoon resultsin high flushing rate in most reservoirs and it retards

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Figure 4. Dominant phytoplankton in reservoirs (state-wise).

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Table 3. Physical-chemical features of Inidan reservoirs (range of values) (from Jhingran, 1990)

Parameter Overall range Productivity rating

Low Medium High

(A) WaterpH value 6.5–9.2 <6.0 6.0–8.5 >8.5

Alkalinity (mg l−1) 40–240 <40.0 40.90 >90.0

Nitrates (mg l−1) tr.-0.93 negligible up to 0.2 0.2–0.5

Phosphates (mg l−1) tr.-0.36 negligible up to 0.1 0.1–0.2

Specific conductivity (µS) 76–474 up to 200 >200

Temperature (◦C) 12.0–31.0 18 18–22 >22

(with minimal stratification)

(B) SoilpH 6.0–8.8 <6.5 6.5–7.5 >7.5

Available P (mg kg−1) 0.05–0.62 <0.3 0.3–0.6 >0.6

Available N (mg kg−1) 1.3–6.5 <2.5 2.5–6.0 >6.0

Organic Carbon (%) 0.6–3.2 <0.5 0.5–1.5 1.5–2.5

colonization by macrophytic communities. Similarly,inadequate availability of suitable substrata also re-tards periphyton growth. By virtue of drifting habitand short turnover period, plankton, constitutes themain link in the trophic structure of the reservoir eco-system. A rich plankton community with well-markedseral succession is the hallmark of Indian reservoirs.Blue-green algae form the mainstay of plankton com-munity in vast majority of the man-made lakes. Theoverwhelming presence ofMicrocystis aeruginosainIndian reservoirs is remarkable. The productive wa-ters of Gangetic plains, Deccan plateau, south TamilNadu and Orissa invariably have good standing cropof Microcystis aeruginosa.A common feature of allthese reservoirs is the bright sunshine, isothermal wa-ter column, klinograde oxygen curve and an extensivecatchment area, draining forested or cultivated landrich in calcium. The examples are Rihand (Natarajanet al., 1982), Nagarjunasagar (Natarajan & Pathak,1983), Amravathy (Sreenivasan, 1970a) and Hirakud(Sugunan & Yadava, 1992). However, the reservoirsof Karnataka and Kerala tend to be oligotrophic andhave poor plankton count with desmids and othergreen algae, as the main constituents. Reservoirs ofRajasthan with scanty rainfall and poor flushing ratefavour macrophytes; despite being productive, they donot harbour blooms ofMicrocystis. Oligotrophic lakesof the northeast have a desmid-dominated planktoncommunity (Figure 4).

Three distinct plankton pulses are reported frommany of the reservoirs, which coincide with post-southwest monsoon (September–November), winter(December–February) and summer (March–May).The southwest monsoon (June–August) flushing dis-turbs and often dislodges the standing crop of plank-ton. However, no sooner the destabilizing effects weanaway (as the dam outlets are closed), the allochthon-ous nutrient input favours an accelerated planktongrowth. As post-monsoon merges into winter, theturbulence decreases and water becomes clean, theplankton community progresses through a series ofseral successions to culminate in a peak. The summermaxima coincide with the drastic drawdown, bringingthe deep, nutrient-rich areas into the fold of troph-olytic zone. High temperature, bright sunlight andrapid tropholytic activities also accelerate the multi-plication of plankton during summer. In some cases,only two pulses (i.e. post-monsoon and summer) areseen. However, the shallow, nutrient rich reservoirs inthe southern tip of the peninsula, by virtue of the fastturnover of nutrients and availability of sunshine andwarmth, sustain a permanent bloom of plankton.

Microcystismultiplies rapidly in peninsular reser-voirs, often reaching blooming proportions. This isan example of a lacustrine biocoenose giving wayto fluviatile ones as a consequence of the impound-ment. Studies have indicated that Chlorophyceae andBacillariophyceae constituted the main componentsof riverine plankton (Sugunan, 1991). On reservoir

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formation and consequent transformation of lotic en-vironment into lentic system, saprophobes disappear,giving room for the rapid multiplication of saproxenes.In the new but favourable environment,Microcystisaeruginosabursts into blooms, outnumbering all otherforms into insignificance. In many reservoirs, orienta-tion of lacustrine and fluviatile plankton can be clearlydiscerned from the composition of plankton in lotic,lentic and the cove sectors. The fluviatile lotic sec-tor, although recording a lower plankton density, oftenshows better diversity and evenness indices, comparedto the lentic and bay sectors (Sugunan, 1991).

In most reservoirs, aquatic macrophytes are totallyabsent or their population is too insignificant to betaken into account, barring some exceptional circum-stances, such as low water renewal, ageing of reservoirand pollution stress. They are mostly restricted to isol-ated patches ofVallisneria maxima, Hydrilla spiralisand mats ofSpirogyra vericellatain protected baysand coves. Yerrakalava in Andhra Pradesh (Sugunan,1995), Ramgarh in Rajasthan (Jhingran,1989a) andSayajisarovar in Gujarat (Ganapati & Pathak, 1969)are examples of macrophytic growth due to low flush-ing rate. Small irrigation reservoirs in Uttar Pradesh,viz., Bachhra (Khan et al., 1990) and Baghla (Sug-unan, 1995) are also known for luxuriant growth ofmacrophytes. Hussainsagar in Andhra Pradesh (Rao,1990) and Mansarovar in Madhya Pradesh (Kulshresh-tha et al., 1992) harbour thick vegetation, whichthrives due to hyper-eutrophication. These reservoirsare well-advanced on their way to swampification.Age of reservoir seems to have an influence on mac-rophyte community. Vanivilas Sagar (Ray, 1969) andMarkonahalli reservoir (Anon, 1998) in Karnatakaformed in 1901 and 1939, respectively, have lux-uriant growth of macrophytes. Similar age-relatedmacrophyte growth can be observed in Yerrakalava(Sugunan, 1995), Ramgarh (Jhingran, 1989a), Hus-sainsagar (Zafar, 1966) Jaisamund and Fatehsagar(Sharma, 1980) reservoirs. Reservoirs of Rajasthanexhibit a seasonal rhythm in aquatic weeds, theirpopulation peaking in summer and declining duringmonsoon.

In small irrigation reservoirs of Karnataka, varietyand biomass of macrovegetation depend, to a large ex-tent, on physiographic divisions and soil types. Thickvegetation, comprising littoral, submerged and emer-gent types, is characteristic feature of the tanks ofcoastal plains and the Malnad region. In the trans-itional zone, between plateau and hills, marked by thepresence of laterite and red soil, the tanks are fertile

and plankton-rich. Weeds are scarce and, wheneverpresent, do not choke the water. In the tanks of blacksoil zones, the weeds are mostly submerged and emer-gent types such asHydrilla sp.,Charasp. andNitellasp. In most of the tanks in the seasonal, red soil area,the vegetation is limited to littoral areas (David et al.,1974).

By providing substrata for a number of insects,molluscs and other invertebrate fauna, macrophytesaugment species diversity of reservoirs. Nevertheless,presence of weeds can be considered as undesirablefrom fisheries point of view. They accumulate largequantities of inorganic nutrients early in the season,depriving the phytoplankton of their share of nutri-ents (Jhingran, 1988). The floating vegetation utilizesthe incident solar radiation for its photosynthesis andmakes it unavailable to the phytoplankton communit-ies. Submerged weeds provide shelter for minnowsand weed fishes which compete with major carps forfood. Excessive growth of macrophytes causes highrate of decomposition of dead plants at the bottom,creating anaerobic conditions. Problems are furthercompounded, if water surface is matted by floating ve-getation, which prevents light penetration. Instances offish mortality in summer under such circumstances arereported from Hussainsagar (Hingorani et al., 1977)and the reservoirs of Karnataka (Raghavan et al.,1977) and Uttar Pradesh (Jhingan, 1988). A majordeleterious effect of weeds is the physical obstructionthey cause to a variety of fishing gear.

The main factors adversely affecting benthic com-munity in reservoirs are the rocky bottom, frequentwater level fluctuations and loss of substrata due torapid deposition of silt and other suspended particles.In spite of this, some reservoirs harbour rich com-munities of benthic invertebrates. Sequence of dom-inance of benthic communities closely follows soilfertility pattern, pre-impoundmentdebris often provid-ing suitable habitats. Benthic community succession,especially that of chironomids, is sometimes used tocharacterize habitat changes. High shoreline develop-ment, variable slopes and vegetation act as favourablefactors for the development of a rich assemblage ofbenthic organisms

Small irrigation reservoirs of the Gangetic basin,such as Bacchra and Baghla, are particularly richin benthic fauna mainly due to rich organic mat-ter in soil and absence of swift changes in waterlevel. In Bacchra, standing crop of benthos registereda steady growth from 490 to 1894 no m−2 duringa 10-year-period (Khan et al., 1990). Baghla has

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a population of benthic invertebrates represented byChironomussp., annelids and molluscs. The deep Ri-hand reservoir in the Ganga basin has a poor benthiccommunity. Reservoirs of Karnataka such as Tun-gabhadra, Markonahalli, Hemavathy, Vanivilas Sagarand Krishnarajasagar have impressive populations ofbenthic organisms, so are the reservoirs of HimachalPradesh and Rajasthan. Trends in Tamil Nadu andMadhya Pradesh are erratic. Local conditions ratherthan a general geo-climatic feature of the area de-termine benthic population density in reservoirs ofIndia. Being a saprophobic, chironomid larvae quicklyfill the niches vacated by the saproxenes during hab-itat transformations. They form the most importantconstituent of benthos from all soil types and geo-graphic locations. Gastropods and annelids form thenext important groups.Viviparus bengalensisenjoyscountry-wide distribution.

Literature on periphyton of the reservoir ecosystemis meager. This community constitutes an importantcomponent of food for the browsing fishes, whichcontribute substantially to total fish biomass of thetropical reservoirs. Apart from the limited littoral re-gion in reservoirs, it is the frequent level fluctuationthat prevents periphyton growth on natural substrata.Significantly, rich periphyton, whenever reported, co-incides with relatively stable reservoir levels. Thereare reports of rich periphyton deposits on anchoredboats and rafts; the fixed substrata either are totallyexposed, when water level decreases or submerged toodeep for the communities to survive, when level goesup. Propensities for rich settling rates of periphytonhave been established through experiments with artifi-cial substrata such as glass slides (David et al., 1975;Jha, 1979; Sugunan & Pathak, 1986).

Ichthyofauna

Although ichthyofauna of a reservoir basically repres-ent the fauna of the parent river system, fish speciesdiversity usually suffers a setback on impoundment.Indian reservoirs, however, preserve a relatively richvariety of fish species (Table 4). Large reservoirs, onan average, harbour 60 species of fishes, of whichat least 40 contribute to commercial fisheries (Jhin-gran, 1990, 1991). The fast-growing Gangetic carps,viz., catla (Catla catla), rohu (Labeo rohita) andmrigal (Cirrhinus mrigala), popularly known as In-dian major carps, occupy a prominent place among thecommercially important fishes. More recently, num-

ber of exotic species have contributed substantially tocommercial fisheries.

The groups enjoying country-wide distribution arethe catfishes, featherbacks, air breathing fishes, mur-rels and minnows. Distribution of Indian major carps,minor carps and mahseers varies according to riverbasins. Fish faunistic diversity of a reservoir at a giventime is the result of the impact of a series of man-madeand natural changes on native fauna of the parent river.Riverine fish fauna are subjected to a series of habitatchanges such as water current, turbidity levels, fishingpressure, loss of breeding grounds and changes in fishfood organisms due to lake formation. Species, whichare sensitive to habitat variables, perish, and the hardyones take advantage of the vacant niches. Formation ofreservoirs has affected a number of riverine fish stocksin India (Table 5).

In many reservoirs, transplantation of fishes fromother basins and introduction of exotic species haveled to further radical changes in species assemblage.Indian major carps are being stocked in reservoirsall over the country for the last three decades andin some instances, they have established themselvesin water bodies far away from their original habitat.Sathanur reservoir in Tamil Nadu has naturalized pop-ulation of catla that contributes 80–90% of the totalcatch. It has eclipsed all indigenous fish fauna includ-ing Labeo fimbriatus, which dominated the scene bycontributing 36% of the catch during the mid-1960s.Similarly, introductions of silver carp in Gobindsagar,common carp in Krishnarajasagar and tilapia (Oreo-chromis mossambicus) in Amaravathy are examples ofman-made changes in fish assemblage.

Some species of fish are known to adapt them-selves to reservoir ecosystem and flourish there tak-ing advantage of the increased biomass of planktonand benthos during the early stages of impoundment.However, most fishes that manage to multiply in thereservoir system are not very high in priority fromcommercial and ecological point of view. Stock ofthe small clupeidSalmostoma phuloandOsteobramavigorsii, which support a flourishing dry fish tradein Nagarjunasagar and Tungabhadra reservoir (Sug-unan, 1995), multiply in much higher scale than theydo in the riverine ecosystem. The catfish,Pangasiuspangasius,which was believed to be a catadromousmigrant, has not only adapted itself to become a res-ident population Nagarjunasagar but also become avery important component of commercial catch. Ra-makrishniah (1994) described many instances, wherereservoirs acted as sanctuaries by citing examples of

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Table 4. Common fish species found in reservoirs of India

Group Species

Indian major carps Labeo rohita, L. calbasu, L. fimbriatus, Cirrhinus mrigala, Catla catla

The mahseers Tor tor, T. putitora, T. khudree, Acrossocheilus hexagonolepis,

Minor carps Cirrhinus cirrhosa, C. reba, Labeo kontius, L. bata, L. dero, L. dussumeri,

Puntius sarana, P. dubius, P. carnaticus, P. kolus, P. dobsoni, P. chagunio, P.

pulchellus, P. jerdoni P. curumuca, Thynnichythys sandhkhol, Osteobrama

vigorsii

Snow trouts Schizothoraxsp.,S. plagiostomus

Large catfishes Wallago attu, Aorichthysaor,A. seenghala, Mystus punctatus, M. gulio,

Pangasius pangasius, Silonia childrenii,

Featherbacks Notopterus notopterus, N. chitala

Air breathing catfishes Heteropneustes fossilis, Clarias batrachus

Murrels Channa marulius, C. striatus, C. punctaus, C. gachua

Minnows Ambassis nama, Esomus dandricus, Aspidoparia morar, Amblypharyngodon

mola, Puntius sophore, P. ticto, Oxygaster bacilla, Laubuca laubuca, Barilius

barilia, B. bola, Osteobrama cotio, Gadusia chapra

Exotic fishes Oreochromis mossambicus, Hypophthalmichthys molitrix, Cyprinus carpio

specularis, C. carpio communis, Gambusia affinis, Ctenopharyngodon idella

Table 5. Fish species affected by dam construction in India

River basin Affected species

Indus Mahseers, snow trouts, Labeo dero, L. dyocheilus, freshwater prawns

Mahanadi Tenualosa ilisha, Puntius sarana, Tor tor, T. mosal, Labeo fimbriatus, L. calbasu,

Rhinomugil corsula, freshwater prawns

Cauveri Puntius dubius, P. carnaticus, Cirrhinus cirrhosa, C. reba, Labeo kontius, L. fimbriatus,

freshwater prawns

Krishna Tenualosa ilisha, Puntius sarana, P. kolus, P. porcellus, P. potail, L. pangusia, L. fimbriatus,

L. calbasu, freshwater prawns

Barilius bola, in Tilaiya, (Damodar),Mystus krishnen-sis, Osteobrama vigorsii, andPseudeutrpius taakreein Nagarjunasagar (Krishna),Thynnichthys sandkholin Nizamsagar (Godavari) Tor khudreeandT. mussal-lah in Shivajisagar (Krishna),Aorichthys seenghala

and Tor putitora in Pong (Beas) and Vallabhsagar(Tapti).

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Fisheries management of reservoirs in India

Phytoplankton is the major primary producer in thereservoir ecosystem and fish productivity depends onthe efficiency of the water body to transform solarenergy into chemical energy. Transformation rate ofthis chemical energy by consumers at different trophiclevels differs considerably from one reservoir to an-other depending on the qualitative and quantitativevariations in the biotic communities. Conversion rateof above 1% from primary producers to fish can beconsidered as good. In an ideal situation, commer-cial species share the ecological niches in such a waythat trophic resources are utilized to optimum. At thesame time, the fishes should belong to a short foodchain to maximize the efficiency of converting theprimary food resources into harvestable materials. Butin reservoirs, such conditions seldom exist.

The major food niches of the Indian reservoirsare the biotic communities comprising phytoplank-ton (Cyanophyceae, Chlorophyceae, Dinophyceae andBacillariophyceae), zooplankton (copepods, clado-cerans, rotifers and protozoans) and benthos (insectlarvae and nymphs, oligochaetes, nematodes and mol-luscs). Significantly, many of the above niches withthe exception of insects, Cyanophyceae and molluscsare shared between Indian major carps and uneco-nomic species, focussing need for controlling thelatter. The ecosystem-oriented management policyplaces due emphasis on trophic strata in terms ofshared, unshared and vacant niches. Two main path-ways, through which primary energy finds its wayto fish, are the grazing and detritus chains (Nata-rajan & Pathak, 1983). Contribution through thesechains leading to transfer of energy to fish level needsto be assessed for determining the species combina-tion, most suited to an ecosystem. Scientific fisheriesmanagement implies utilization of the available foodchain in reservoir by employing the right kinds ofspecies, fish stock monitoring through adjusting thequantum of fishing effort and mesh size, and adoptingconservation measures.

Assessment of yield potential

Several methods are in vogue to assess the fishery po-tential of reservoirs by deriving equations based onarea, depth, catchment area and the chemical paramet-ers of soil and water. Morpho-edaphic index (MEI) is amethods that uses simple, easily available parametersreflecting morphometric as well as chemical charac-

Table 6. Estimated fish yield (using MEI method) and actualyield in seven reservoirs in India

Reservoir Estimated yield Actual yield(kg ha−1) (kg ha−1)

Pong (Himachal Pradesh) 33 32Ukai (Gujarat) 67 46Stanley (Tamil Nadu) 51 12Nagarjunasagar (Andhra Pradesh) 48 06Rihand (Uttar Pradesh) 27 05Gandhisagar (Madhya Pradesh) 52 08Getalsud (Bihar) 68 02

ters. Relationships between MEI and catch are basedon some common characteristics for sets of lakes thatpossess a certain number of limnological conditions.These are (1) ionic composition (dominated by thecarbonate-bicarbonate system), (2) water body (notdystrophic), (3) volume (not noticeably fluctuating)and (4) similar thermal regime. Indian reservoirs, byand large, fulfil all these conditions except the oneon fluctuations in volume. Henderson & Welcomme(1974) have calculated morpho-edaphic index and fishyield potential for the African lakes as:

MEI = Specific conductivity (µS)

Mean depth (m)and

Fish yield (kg ha−1) = 14.3136 MEI0.4681.

Although the Asian reservoirs are known to have alower yield potential, till an Indian model is derived,this formula can be applied to the Indian reservoirs toobtain a rough indication of productivity. However, theactual fish yields from most of the reservoirs are muchlower than the potential estimated using the formula(Table 6).

Capture fisheries, culture-based fisheries andenhancement

Classification of reservoirs into small, medium andlarge, based on size, has limitations in setting manage-ment guidelines. The major consideration in choosinga particular management option is the degree, at whichthe environmental parameters and fish stock can bemanipulated to increase the yield rate. A very usefulcriterion to distinguish the norms of capture fisher-ies from those of culture fisheries is the level ofhuman intervention in ecosystem management. In in-tensive aquaculture, the manager exercises a certainlevel of freedom in modifying the ecosystem both in

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terms of environment and biotic communities. On theother hand, in capture fisheries, wild untended popu-lations are harvested with little option to modify theenvironment.

Management of medium and large reservoirs in In-dia can be considered as more akin tocapture fisheries.Although most medium and large reservoirs in variousstates are regularly stocked, their fishery depends, to alarge extent, on the wild or naturalized fish stock. Fish-eries of the reservoirs like Bhavanisagar, Sathanur,Krishnagiri, Malampuzha, Hirakud, Nagarjunasagar,Rihand and Tungabhadra are dependent on naturalrecruitment (Sugunan, 1995). Thus, management ofthese reservoirs are basically oncapture fisherieslineswith some modes of stock and speciesenhancement.Conversely, small reservoirs in India depicts a totallydifferent picture, where the impact of stocking is morediscernible. Since it is essentially a stock and recapturesystem, the small reservoir fishery management areculture-based fisheries. However, there is no thumbrule to differentiate the two systems based on reser-voir size. Fishing conditions, shallowness of reservoirand natural recruitment are the major factors that de-termine whether capture or culture-based fisheries isfollowed.

The key management parameters of culture-basedfishery are stocking density, size at stocking, size atcapture and growout period. Fisheries enhancementis a process, by which qualitative improvement isachieved from water bodies through exercising spe-cific management options. The common modes ofenhancement, which are relevant to inland water bod-ies of India, arestock enhancement(increasing thestock), species enhancement(inducting new speciesto broaden the catch structure) andenvironmentalenhancement(enriching the water quality through ar-tificial eutrophication).

Stock enhancement

Stocking of reservoirs with fingerlings of economic-ally important, fast growing species to colonize all thediverse niches of the biotope has become one of thenecessary prerequisites in reservoir fishery manage-ment. The basic principles that should be followed inselecting a species to be stocked are (Jhingran, 1988):

1. The planted species should find the environmentsuitable for survival and growth.

2. It should be a quick growing, highly productiveherbivorous fish with shorter food chain and higherefficiency of food utilization.

3. Number of them to be planted should be such thatfood resources of the ecosystem are fully utilizedand densest population maintained consistent withnormal growth.

4 Stock should be readily available without majorshift in the cost involved in its transportation.

5. Cost of stocking and managing the species mustbe less than the benefits derived from stocking andmanagement.

However, evaluation of an array of factors like thebiogenic capacity of the environment, growth rateof the desired species and the population density asregulated by predatory and competitive pressures areneeded to be evaluated before stocking. The policiesand guidelines currently available on the subject arestill erratic and even arbitrary.

Stocking rateA large country like India, with too many water bod-ies to be stocked, has inadequate state machinery tomeet the stocking requirements of all its reservoirs.This has resulted in under-stocking of the reservoirs.Stocking densities need to be fixed for individual waterbodies or a group of them sharing common character-istics such as size, presence of natural fish populations,predation pressure, fishing effort, minimum market-able size, amenability to fertilizing and multiplicityof water use. The main considerations in determin-ing the stocking rate are growth rate of individualspecies stocked, mortality rate, size at stocking andgrowing time. Based on the National Consultation onReservoir Fisheries (Sugunan, 1997a), Government ofIndia has recently adapted the following formula (Wel-comme, 1976) to calculate the stocking rate for smallreservoirs:

S =(q · PW

)e−z(tc−t0)

whereS is the number of fish to be stocked (in noha−1), P is the natural annual potential yield of thewater body (kg ha−1), q is the proportion of the yieldthat can come from the species in question,W is themean weight at capture,tc is the age at capture,t0 isthe age at stocking and−z is the total mortality rate.P can be estimated through MEI method (men-

tioned above) and the range of mortality rates can befound out from the estimated survival rate. Table 7

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Table 7. Calculated stocking density at dif-ferent levels of mortality (from Welcomme,1976)

Annual survival −z Estimated stocking

(% ) density (no, ha−1)

50 0.7 805

37 1.0 1087

22 1.5 1792

13 2.0 2955

Table 8. High yields obtained in some small reservoirs due to stockenhancement (from Sugunan, 1995)

Reservoir State Stocking rate Yield

(no. ha−1) (kg ha−1)

Aliyar Tamil Nadu 353 194

Meenkara Kerala 1226 107

Chulliar Kerala 937 316

Markonahalli Karnataka 922 63

Gularia Uttar Pradesh 517 150

Bachhra Uttar Pradesh 763 140

Baghla Uttar Pradesh – 102

Bundh Beratha Rajasthan 164 94

illustrates calculation of stocking rates using the for-mula given above, whenP = 200 kg ha−1, q = 1, W =0.5 kg andtc−t0 is 1. The model assumes insignificantbreeding by stocked population and therefore appliesmainly to total cropping situations i.e. those in whichfish are caught below their minimum size for maturity,those whose natural reproduction does not take placeand those where water body is not permanent. It showsthat stocking density, which depends on natural condi-tions of productivity, growth and mortality, are verysensitive toZ. Because of the very large numbers offry needed, this formula may have very limited utilityin large reservoirs.

Impact of stocking in small reservoirs

Despite the lack of any national policy and soundscientific advice, stocking efforts made in India havebeen very effective in improving the yield from smallreservoirs. This is because of the fact that success inmanagement of small reservoirs depends more on re-capturing the stocked fish rather than building up abreeding population. The smaller water bodies havethe advantage of easy recapture and stock monitoring.

The basic tenets of stocking policy (Sugunan, 1995)are:

1. Selection of the right species, depending on thefish food resources available in the system.

2. Determination of a stocking density on the basis ofproduction potential, growth and mortality rates.

3. Proper stocking and harvesting schedule includingstaggered stocking and harvesting, allowing max-imum grow out period, taking into account thecritical water levels.

4. In small irrigation reservoirs with open sluices, theseason of overflow and the possibilities of waterlevel falling too low or completely drying up arealso taken into consideration.

Some success stories of stocking in small reservoirsare listed in Table 8.

Environmental enhancement

The improvement of nutrient status of water bodyby selective input of fertilizers is a very commonmanagement option adopted in intensive aquaculture.If similar environmental enhancement is adopted insmall reservoirs, stocks can be maintained at levelshigher than the natural carrying capacity of the ecosys-tem. However, a careful consideration of the possibleimpact on the environment is needed before this optionis resorted. Scientific knowledge to guide the safe ap-plication of this type of enhancement and the methodsto reverse the environmental degradation, if any, arestill inadequate. On account of all these, this is nota very common management tool. China is known tohave used this instrument in a big way to augmentproduction from small reservoirs. Taking cue fromChina, Cuba has manured small reservoirs using bothorganic and inorganic fertilizers. This is also practicedselectively in the community water bodies of Thailand(Sugunan, 1997b).

Sreenivasan & Pillai (1979) attempted to improveplankton productivity of Vidur reservoir by applyingsuper-phosphate with highly encouraging results. Asan immediate result of fertilization, phosphate con-tent of water increased from nil to 1.8 mg−1 andthat of soil from 0.242 to 0.328%. Experiments werealso conducted with urea in the same reservoir. Aftertwo successive applications, significant limnologicalchanges took place including the presence of free car-bon dioxide and decrease in pH and dissolved oxygenat bottom. As a direct benefit from fertilization, a 50%

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increase in fish production, and a three-fold increasein the size of catla, rohu, mrigal,L. fimbriatusandL.calbasuwere achieved.

Application of lime was tried in some uplandlakes for amelioration of excessive carbon dioxideand acidity at the bottom (Sreenivasan, 1971). Thismeasure, together with application of super-phosphatein Yercaud lake, raised the pH from 6.2 to 7.3 anddecreased carbon dioxide from 38 to 6.5 mg l−1

at the bottom. There were corresponding increasesin species number and biomass of plankton. Sim-ilar experiments are reported from Kyrdemkulai andNongmahir in Meghalaya (Sugunan & Yadava, 1991a,b) from Natkara reservoir in Madhya Pradesh (Su-gunan, 1995). Chinese experience in fertilizing thesmall water bodies to increase productivity, especiallythe Shishantou reservoir has been very encouraging(Yang et al., 1990).

Modeling approach in culture-based fisheries

Available models on culture-based fisheries of smallreservoirs have clearly confirmed that production isa function of fishing mortality and stocking density.If some standard variables on population parameters,such as thedensity-dependent growth, size dependentmortality and weight–length relationshipare known,optimum stocking density and fishing mortality can bearrived at. Thus, a desired balance between stockingrate, population density and growth can be maintained.Lorensen (1995). Illustrated a model on culture-basedfisheries of Thailand (Figure 5). This tool has beeneffectively used in many countries (Sugunan, 1997b)to make necessary adjustments in fishing effort.

Management of medium and large reservoirs

Since large and medium reservoirs are to be developedon the principles of capture fisheries, the main ac-cent of management is on conservation of habitat inorder to allow natural recruitment and growth of thetarget species. Stock monitoring is achieved throughmaneuvering of fishing effort and following mesh sizeregulations. Introduction is resorted to correct im-balances in species spectrum, and stocking is doneas a temporary measure to compensate for recruit-ment failure. Stocking attempts in medium and largereservoirs are successful only when the stocked fishesbreed and propagate themselves. Catla stocked inSathanur, Gandhisagar and Ukai also led to increase

in yield, primarily because of its breeding success(Sreenivasan, 1984). In sharp contrast, in a number ofreservoirs like Nagarjunasagar, Bhavanisagar, Krish-nagiri, Malampuzha and Peechi, the fish did not makean impact, as it failed to breed. In Nagarjunasagar, an-nual stocking at the rate of 50 000–833000 fingerlingsof catla, rohu and mrigal during 1970s had little impacton the catch structure, as none of the stocked fishescould breed and contribute to recruitment (Sugunan,1995).

Species enhancement

Species enhancement aims at augmenting the speciesrange by adding fish species from outside with a viewto colonize all the diverse niches of the biotope forharvesting maximum sustainable crop. It can be juststocking of a new species or introductions (whichmeans one time or repeated stocking of a species out-side its range of natural distribution). In India, thereare no restrictions on fish transplantation on trans-basin basis. Catla, rohu and mrigal are being stockedin the peninsular reservoirs for the last five decades.This is despite the fact that the peninsular rivers havehabitats, distinctly different from those of Ganga andBrahmaputra to which these fishes are indigenous. Insome of the south Indian reservoirs, they have es-tablished breeding populations. The hallmark of theIndian policy on stocking (actually introductions incase of peninsular reservoirs) is the heavy dependenceon Indian major carps. There is evidence that the Gan-getic major carps have affected the species diversity ofpeninsular cyprinids (Sugunan, 1995).

Introduction of exotics

In spite of an already rich and diverse fish genetic re-source of India, more than 300 species have alreadybeen introduced into the country (Jhingran, 1989b).While a vast majority of them are ornamental fishesconfined to aquaria, some like tilapia (Oreochromismossambicus), silver carp (Hypophthalmichthys mo-litrix ), grass carp (Ctenopharyngodon idella) and threevarieties of common carp (scale carpCyprinus car-pio communis, mirror carpC. carpio specularisandleather carpC. carpio nudus) have been broughtfor aquaculture purposes. In recent years, the big-head carp (Aristichthys nobilis), Nile tilapia (Oreco-chromis niloticus) and African catfish (Clarias gar-iepinus) have become popular among aquaculturists.The tilapia,O. mossambicuswas stocked in reservoirs

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Figure 5. Production as a function of fishing mortality and stocking density for a gear selection length of 30 cm (After Lorenzen, 1995).

of south India during the 1960s. Jhingran (1991) repor-ted a gradual decline in size of tilapia in reservoirs ofTamil Nadu and Kerala over the years. Barring Kerala,the fish has low consumer preference. Today, fisherymanagers in India do not preferO. mossambicusas acandidate for stocking (Sugunan, 1995).

An experimental consignment of 239 fingerlings ofsilver carp was stocked in Kulgarhi reservoir (Mad-hya Pradesh) in 1969, 10 years after its introduction.Based on the recapture of 8 specimens, growth ratesranging from 597 mm in 783 days and 404 mm in 293days have been reported (Rao & Dwivedi, 1972). Sim-ilar trials in Getalsud reservoir (Bihar) showed growthrate of 2.2–5.79 g, d−1. In Gobindsagar reservoir(Himachal Pradesh), the fish, after an accidental in-troduction, formed a breeding population and broughtabout a sharp increase from 160 t in 1970–71 to morethan 1000 t at present. Jhingran & Natarajan (1978)pointed out that silver carp, being cold water fish in-troduced in India, consumed food much in excess andgrew faster as expected of a true poikilotherm. A sim-

ilar latitude-induced change was noticed, as it maturedin one year, compared to 5 years in China. Whileevaluating the co-existence of silver carp and catla,Karamchandani & Mishra (1980) established that thetwo fishes shared a common niche and competed witheach other for food in the reservoir ecosystem. Jhin-gran & Natarajan (1978) cautioned against introducingthe fish in Indian reservoirs connected to major riversystems, as it might adversely affect catla and otherprecious indigenous carps of the country. However, itis significant to note that despite its entry into a num-ber of reservoirs, by accident or otherwise, silver carpfailed to breed anywhere except in Gobindsagar.

While the Bangkok strain of common carp isstocked in reservoirs of the plains, the European strainis preferred in the temperate zones and at high alti-tudes. But their performance in reservoirs was erratic,despite heavy stocking. Being a sluggish fish, itschances of survival in predator-dominated reservoirsare very poor. They are not frequently caught in apassive fishing gear like gill net due to its slow move-

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ment and bottom dwelling habit. In Nagarjunasagar,despite regular stocking for 13 years, common carpdid not form a part of the fishery (Sugunan, 1995).A more important disqualification of common carpis its propensity to compete with some indigenouscarps likeCirrhinus mrigala, C. cirrhosaandC. reba,with which it shares a food niche. Instances of com-mon carp causing the decline ofCirrhinus spp. areGirna (Valsankar, 1987) and Krishnarajasagar (Sug-unan, 1995) reservoirs. Detailed accounts of mirrorcarp affecting the survival of native fish species inGobindsagar reservoir, upland lakes of Kashmir andKumaon Himalayas (Schizothoraxspp.), and Loktaklake in the northeast (Osteobrama belangiri) are givenby Sugunan (1995).

Removal of predators and minnows

Checking these unwanted populations is a very diffi-cult management problem, especially in large reser-voirs. Repeated use of gill nets of appropriate meshsize, use of long lines and traps are suggested forcontrol of the uneconomic and undesirable popula-tions. David & Rajagopal (1969) reported that thegiant shore seine,alivi, helped in reducing the cat-fish population in Tungabhadra reservoir by 76–81%.Alivi also removes minnows in large numbers. Re-cent findings of Kartha & Rao (1990) with regard tothe efficacy of trawling in checking predators and un-economic fishes are of interest. Bottom trawling inGandhisagar caught 64–91% of unwanted fishes andthis has been recommended as a method to crop thepredators and carp minnows. Natarajan (1979b) sug-gested biological control of trash fishes by stockingtwo euryhaline species viz.,Megalops cyprinoidesandLates calcarifier.Since these predatory fishes do notbreed in freshwater, they cannot go out of control.

Acknowledgements

The author is grateful to the Director, Central InlandCapture Fisheries Research Institute (ICAR), Barrack-pore, India, for providing facilities for the preparationof this paper.

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